Measurements of the transmission and the reflectance spectra of whole blood exhibit a different dependence of the thickness of the blood layer. Whereas the transmission spectra decrease at all wavelengths, in the reflectance two ranges with different behaviour have to distinguish. In the absorption dominated range between about 500 and 600 nm, there is only a marginal influence of the thickness of the blood layer. In contrast, in the scattering dominated range higher than about 600 nm the reflectance increases if the thickness of the blood layer increases. Proofed by Monte Carlo simulation, the reflected light, which will be measured in ophthalmologic devices, consists both of light which penetrates a vessel once and will be reflected at the background and of light which is internally reflected at the blood column. Only in small vessels (diameter < 50 microm) the transmitted part dominates. With increasing diameter of the vessels the internally reflected light dominates. Especially under the conditions of a confocal laser scanner, the transmitted part of light is always small. As the measured light consists of both parts of light, a new model for the approximation of the measured reflected light is given, containing an expression for the transmitted and for the internally reflected light. The model was applied for the approximation of the reflectance of blood in a cuvette with an coloured background, delivering sufficient exact result. The oxygen saturation was determined in 256 measurements of 30 healthy persons using an ophthalmo imaging spectrometer. The mean arterial oxygen saturation was 91 +/- 5%, the mean venous oxygen saturation was 59 +/- 9% and the arterio-venous difference in the oxygen saturation was 35%, which is in good agreement with the arterio-venous difference known in the brain. First measurements of the oxygen saturation in age-related macular degeneration point to partly different pathological changes of the retinal microcirculation in dry and in wet AMD.